1. Field of the invention
The present invention is generally concerned with synchronizing mobile terminals in a cellular mobile radio network and in particular with a method of accelerating the synchronization of a mobile terminal in a mobile telephone system.
2. Description of the Prior Art
GSM type mobile radio systems are cellular networks in which each cell is the radio coverage area of a base transceiver station (BTS). Each cell can be identified by a mobile terminal from a frequency on which the base transceiver station broadcasts control information to all terminals present in the cell and adjacent cells. In the GSM system the number of frequencies that can be used is 124.
When a terminal wishes to identify itself to the network, it must begin by identifying the cell in which it is located. This is achieved by listening in succession to all the frequencies provided by the system and determining that received at the highest power. This action ("measurement campaign") can take between three seconds and five seconds.
The terminal then attempts to synchronize to the base transceiver station received at the highest power, and if it fails to do so it attempts to synchronize to the other base transceiver stations in decreasing power order until it is able to synchronize to one of them.
The measurement campaign and synchronization procedure is undertaken systematically when the terminal is switched on or when the terminal user requests to change networks. However, it is also started in response to any loss of synchronization, for example if the radio link between the base transceiver station and the terminal is broken because the vehicle in which the terminal is located enters a tunnel, or more generally because the terminal is unable to synchronize because of poor radio reception conditions. Unfortunately, during such synchronization, the terminal is unavailable for the duration of the measurement campaign (between three seconds and five seconds) preceding synchronization proper. Terminal users find that this unavailability time is too long and this leads to user dissatisfaction.
For this reason one object of the invention is to provide a method of synchronizing a mobile terminal which in favorable cases avoids the need for any measurement campaign.